You wouldn't be human if you've never used the excuse that you don't have enough time to fit a workout into your day.

Finding a full hour to exercise can be genuinely tricky. Add the time it takes to shower afterwards and it's almost impossible to fit a lunchtime session into a working day. But do you really need an hour to get it done?

"To be honest, unless you're training for an endurance event, doing an hour of exercise is actually almost overtraining," health coach and personal trainer Blake Worrall-Thompson, who whipped Bachelorette Sam Frost into shape for the show, told The Huffington Post Australia.

"Forty-five minutes is actually the ideal amount of time, but anything from 10-30 minutes is much better than a full hour. Training for an hour can actually increase your cortisol (stress hormone) to unhealthy levels."

"While 45 minutes is the ideal, for guys who are looking to increase testosterone -- which is important for muscle -- and for females who are looking to drop body fat, you need to keep your cortisol levels in check-- anywhere between 10-30 minutes is still really effective if the intensity is there. The intensity is what is key. What people should be more worried about is the intensity at which they are training, as opposed to how long they are working for," Worrall-Thompson said.

So taking into account the fact that intensity is key for shorter sessions, how exactly do we know if were working out hard enough?

"If you have a heart rate monitor, that is a great way to determine how hard you're working," Worrall-Thompson said.

"Ideally you want to be working in the 75th percentile of your maximum heart rate. To determinate our maximum heart rate, minus your age from 220. So if you're 40 years old that would be 180. Ideally you'd like to be working out at 75 percent of that, so around 140 beats per minute or above. That's a great indication that you are training at a good intensity."

No fancy tech gadgets on hand? No worries -- you can measure it without a monitor, as long as you are honest with yourself.

"If you don't have a heart rate monitor then you can use a measure called Rate of Perceived Exertion (known as RPE), which is essentially how out of breath you are. If you can hold a conservation then you're probably not training at a higher enough intensity. Of course anything is better than nothing, but if you want bang for buck in terms of results, you want to be working at a seven or above out of 10. Seven or above is essentially not being able to hold a conversation," Worrall-Thompson said.

With the key to a short workout being hard and not long, does it matter what type of exercise you're actually doing?

"When aiming to get the most out of your short workout, as well as ensuring the intensity is there, you need to consider what muscles groups you're working. If someone is spending half an hour working small muscles such as their triceps and biceps they probably aren't going to get as much out of it," Worrall-Thompson said.

"If you're really looking for the best fat burner in that short amount of time, you want to engage the biggest muscles. You're looking to focus on your back, chest and leg muscles. So you might do squats for 30 seconds, then push-ups for 30 seconds, then repeat. Those are big exercises that are going to burn a lot of calories and build good quality lean muscle tissue. If you wanted to do cardio you could easily do 20 minutes of good quality cardio if it is interval training at the right intensity."

It's pretty simple. If you're uncomfortable, you're in a good place.

"This isn't what most people want to hear, but if you're doing interval training for 20 minutes at the right intensity, you should be looking at the clock after 10 minutes hoping for it to be over soon. It's essentially all about a mind shift to work toward intensity instead of a timeframe," Worrall-Thompson said.